1. Field of Invention
The present invention pertains generally to analog circuitry and more specifically to seismic line amplifiers for conditioning geophone signals.
2. Description of the Background
Seismic exploration has been used for some time to locate various geostructures within the earth's crust. Seismic Exploration has typically been carried out by inducing vibrational waves in the earth and detecting the reflection of these vibrational waves at a number of locations on the surface. The detected signals from the geophone are transmitted to a data collection device which processes the information to generate a graphic depiction of the detected seismic waves.
Various disadvantages and limitations are associated with this conventional manner of collecting seismic data. For example, the seismic signal generated by the geophone is an extremely low amplitude signal which must be transmitted over miles of cable to the data collection device which typically comprises a mobile data collection unit. The signal-to-noise ratio of the seismic geophone signal received at the data collection unit is extremely low because of the extremely low amplitude of this transmitted signal. Additionally, attenuation and noise pick up which occurs during the transmission of the low amplitude signal further decreases signal-to-noise ratio of the signal received at the data collection unit. Linear amplification of such low amplitude signals in the presence of noise is difficult to achieve.
Other problems are also apparent. The resolution of data which can be obtained from the seismic geophone signal is dependent upon the frequency response of the signal. Higher frequency seismic signals contain higher resolution data capable of resolving smaller geostructures, than lower frequency seismic geophone signals. For example, seismic geophone signals in the frequency range of 100 to 147 hertz contain considerably higher resolution information than the same seismic geophone signals at frequencies below 100 hertz. The inherent natural filtering effects (Q) of the earth tend to attenuate the higher frequency signals in a non-linear manner. In other words, the earth has an attenuation factor (Q) which increases non-linearly with frequency. As a result, the higher frequency signals, which contain higher resolution information, are attenuated to a much greater extent than lower frequency signals. Consequently, the resolution which can be obtained from the seismic geophone signals is limited by the natural earth filtering effects (Q). In the process of amplifying the seismic geophone signal at the data collection unit, the dynamic range of the seismic geophone signal is limited at higher frequencies due to the attenuation of the higher frequency signals. Consequently, the data contained within the higher frequency signals is de-emphasized, or lost, in the amplified seismic geophone signal.
A search was performed on the present invention and the following patents were selected showing the closest patented art in the field:
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ Montgomery, Jr. 3,699,325 Oct. 17, 1972 Harris et al. 4,321,675 Mar. 23, 1982 ______________________________________
U.S. Pat. No. 4,321,675 issued to Harris et al. on Mar. 23, 1982 discloses a method and apparatus for increasing the dynamic range of a data acquisition system at low frequencies without reducing frequency gain. Harris et al. discloses prior art having a dual input 1, 2 which is applied to a preamp 3 producing a dual output 5, 6 applied to a common mode rejection differential amplifier 7 to generate a single output 8. Single output 8 is applied to a low cut filter 9. The output 10 of low cut filter 9 is applied to a high cut filter 11 which comprises an anti-alias filter having approximately 48 dB per octave attenuation. This output is then applied to a notch filter 13 and to a low dc drift amplifier 15 for application to a multiplexer device 17. Harris et al. discloses the use of a variable gain amplifier disposed between the low cut filter and the high cut filter to expand the dynamic range of the overall line amplifier system by decreasing the amount of noise which is added to the circuit by the high cut filter.
U.S. Pat. No. 3,699,325 issued to Montgomery on Oct. 17, 1972 discloses a time shared instantaneous gain ranging amplifier which is used for a plurality of seismic data channels. The Montgomery system uses a preamplifier 10 which receives the output of the geophones or other similar devices. The output of the preamplifier 10 is applied to optional filters 12 which can include high and low pass filters and a 60 hertz notch filter. The output of the optional filters 12 is then applied to anti-alias filter 14. Preamplifier 10 comprises a differential amplifier which converts the signal from common mode to normal mode. Both the Harris et al. and Montgomery patents are specifically incorporated herein by reference and are made a part of this disclosure for all that they disclose.
The Harris et al. and Montgomery patents do not disclose a signal conditioning circuit which can be located proximate to a geophone in the field and operate with low power consumption from a battery power supply. Additionally, flexibility has not been provided to optimize parameters of the seismic line amplifier for various geologies.
Moreover, the prior art has failed to show a geophone line amplifier which is capable of producing a substantially flat frequency response despite natural earth filtering effects. Although this problem was generally addressed in a paper entitled "How High is High Resolution" at the 46th Annual SEG Meeting, Oct. 24-28, 1976, Houston, Texas, by John B. Farr, which is specifically incorporated herein by reference for all that it discloses, there is no disclosure in the Farr paper of any specific manner of obtaining a substantially flat response. In fact, Farr even suggests that a flat response would not be advantageous. Farr suggests three different ways of generating a flat response but concentrates primarily on the use of a accelerometer as a transducer to detect seismic signals in a non-linear manner. Since the accelerometer has a response which increases with frequency, the output of the accelerometer tends to offset the Q filtering effects of the earth. However, the response of the accelerometer cannot be easily changed to match the Q filtering effects of the earth which vary greatly with soil conditions and geostructures within the earth. Consequently, accelerometers do not provide the flexibility or the preciseness required to produce a flat frequency response for differing geologies.
Consequently, although the effects of natural earth Q filtering have been known since Lord Raleigh's time (19th Century), to date, no system exists which has the capabilities and flexibilities of generating a flat frequency response for varying geologies.